In recent years, restrictions on chemical substances harmful to the environment have become increasingly stringent, and the restrictions on solder materials used to join electronic parts or the like to a substrate are no exceptions. Pb has been used as a major component of solder materials since old times, but has already been designated as a restricted substance by, for example, RoHS Directive. Therefore, solders containing no Pb (Pb-free solders) have been actively developed.
Solders used to join electronic parts to a substrate are broadly divided into high-temperature solders (about 260 to 400° C.) and low- and medium-temperature solders (about 140 to 230° C.) based on their service temperature limits. As for low- and medium-temperature solders, Pb-free solders mainly containing Sn have already been practically used. For example, Patent Document 1 discloses a Pb-free solder alloy composition containing Sn as a major component, 1.0 to 4.0 mass % of Ag, 2.0 mass % or less of Cu, 0.5 mass % or less of Ni, and 0.2 mass % or less of P. Further, Patent Document 2 discloses a Pb-free solder alloy composition containing 0.5 to 3.5 mass % of Ag, 0.5 to 2.0 mass % of Cu, and the balance being Sn.
On the other hand, Pb-free solder materials for high-temperature applications are under development in various organizations. For example, Patent Document 3 discloses a Bi/Ag brazing filler material containing 30 to 80 mass % of Bi and having a melting temperature of 350 to 500° C. Further, Patent Document 4 discloses a solder alloy obtained by adding a binary eutectic alloy to an eutectic alloy containing Bi and by further adding an additive element thereto, and describes that this solder alloy is a quaternary or higher solder, that is, a multi-component solder, but it is possible to adjust its liquidus-line temperature and to reduce variations in composition.
Patent Document 5 discloses a solder alloy obtained by adding Cu—Al—Mn, Cu, or Ni to Bi, and describes that when such a solder alloy is used to join a power semiconductor device having a Cu surface layer to an insulator substrate having a Cu surface layer, an undesired reaction product is less likely to be formed at a joint interface between the solder and each of the Cu layers so that the occurrence of defects such as cracks can be suppressed.
Patent Document 6 discloses a solder composition containing, based on the total mass of the solder composition, 94.5 mass % or more of Bi as a first metal element, 2.5 mass % of Ag as a second metal element, and a total of 0.1 to 3.0 mass % of at least one selected from the group consisting of 0.1 to 0.5 mass % of Sn, 0.1 to 0.3 mass % of Cu, 0.1 to 0.5 mass % of In, 0.1 to 3.0 mass % of Sb, and 0.1 to 3.0 mass % of Zn as a third metal element.
Patent Document 7 discloses a Pb-free solder composition containing a Bi-based alloy containing at least one of Ag, Cu, Zn, and Sb as an accessory component and 0.3 to 0.5 mass % of Ni. Patent Document 7 further describes that this Pb-free solder has a solidus-line temperature of 250° C. or higher and a liquidus-line temperature of 300° C. or less. Further, Patent Document 8 discloses a binary alloy containing Bi, and describes that this binary alloy has the effect of suppressing the occurrence of cracks in the inside of a soldering structure.
Patent Document 9 discloses a Bi alloy having a melting temperature of 270° C. or higher and containing 0.2 to 0.8 mass % of Cu and 0.2 to 0.02 mass % of Ge. Patent Document 10 discloses a Bi alloy having a solidus-line temperature of at least 262.5° C. and containing 2 to 18 mass % of Ag and 82 to 98 mass % of Bi. Patent Document 11 discloses a Bi alloy having a solidus-line temperature of 260° C. or higher and containing at least 80 mass % of Bi.
Patent Document 12 discloses a Bi—Sn-based solder paste and an article joined by means of the solder paste, in which the solder paste contains 30 mass % or more of Bi in the form of metal alloy powder. The solder paste provides higher joint strength after joining and produces no void even though an object to be joined includes Au. Patent Document 12, for example, discloses a solder paste containing 30 to 98 weight % of Bi, 0.01 to 0.5 weight % of either Al or Mn, and the balance being Sn.
Patent Document 13 discloses a solder paste including a solder powder having bismuth or an alloy mainly containing bismuth and having a solidus-line temperature of 250° C. or higher and a liquidus-line temperature of 370° C. or less, a thermoplastic resin to be melted at a temperature higher than the solidus-line temperature of the solder powder and having a function to support the strength of the solder by remaining after soldering, and a flux.